This invention relates to an improved intermediate support device for supporting an intermediate portion of a feed screw shaft in order to avoid a critical speed of the feed screw of a long support span used, for example, in a high-speed table feed device.
During a high-speed feed of a table, such as a mounter in a machine tool or a semiconductor producing apparatus, by a table feed device provided with a feed screw shaft, when the cycle of rotation of the screw shaft coincides with the natural frequency of the screw shaft, a resonance occurs, and this is dangerous. This critical speed is inversely proportional to the square of the distance between support points of the screw shaft, and therefore in the case where the table stroke is long, the support span of the screw shaft is long, so that the critical speed is abruptly lowered. With a higher feed speed, a higher rotational speed of the feed screw has been used, and in some cases a critical speed range has been used. In such a case, it is known to provide an intermediate support device to shorten the support span of the screw shaft to increase the natural frequency.
One conventional intermediate support device of this type is disclosed, for example, in Japanese Laid-Open Utility Model Application No. 2-4737 proposed earlier by the Applicant of the present application. In this device, a nut (which is mounted integrally on a table) is threadedly engaged with a screw shaft rotatably supported at opposite ends thereof by bearings. A pair of front and rear intermediate supports, having two feed screw support portions (slide bearings) which are movable in the direction of the axis of the screw shaft and receive the radial load of the screw shaft, are provided respectively on the opposite sides (front and rear sides) of the nut in the direction of the axis of the nut, and are connected together by a connecting rod. A pair of linear guide device are provided respectively on the opposite (right and left) sides of the screw shaft in parallel relation, and the table is supported by sliders movable along guide rails of the linear guide devices, and the intermediate supports are also supported by other sliders movable along the common guide rails.
A nut which is moved by the rotation of the feed screw is disposed between the feed screw support portions of the pair of intermediate supports. For feeding the table, the nut is first moved in the feed direction by a distance equal to the distance between the intermediate supports, so that the nut is abutted against one of the intermediate supports, and the two interconnected intermediate supports are pushed and moved in unison by this nut.
For returning the table, the nut is moved in the opposite direction by a distance equal to the distance between the intermediate supports, so that the nut is abutted against the other intermediate support, and then is moved, together with the two intermediate supports, to a stop position.
For stopping the nut, one intermediate support is engaged with a limit switch, provided at the end thereof, to operate this switch to stop a rotation drive device, thereby stopping the nut. When the rotation drive device is stopped, the rotation of the screw shaft is stopped. The nut, the table and the intermediate supports are stopped simultaneously when the rotation of the screw shaft is stopped.
In the above conventional example, however, one of the intermediate supports is abutted against the nut, so that the nut is moved together with the intermediate supports. Therefore, there has been encountered a problem that the high-speed movement of the table is prevented by the intermediate supports, so that the speed of movement is slow.